Field of the Invention
The invention relates to a steam turbine having a high-pressure turbine section and a medium-pressure turbine section fluidically connected to the high-pressure turbine section.
Known steam turbines are classified as action turbines (also called "constant-pressure" turbines) and reaction turbines (also called "excess-pressure" turbines). They have a turbine shaft with moving blades disposed on it and have an inner casing with guide blades disposed between axially spaced moving blades.
In the case of a constant-pressure turbine, the entire energy gradient is converted essentially into kinetic flow energy in he ducts that are narrowed by the guide blades. During the process, the velocity rises and the pressure falls. In the moving blades, the pressure and relative velocity remain essentially constant, being achieved through ducts having a uniform clear width. Because the direction of the relative velocity changes, action forces occur that drive the moving blades and, thus, cause rotation of the turbine shaft. The magnitude of the absolute velocity decreases considerably when the flow passes around the moving blades, resulting in a flow that transfers a large part of its kinetic energy to the moving blades and, therefore, to the turbine shaft.
In the case of an excess-pressure turbine, only part of the energy gradient is converted into kinetic energy when the flow passes through the guide blades. The rest of the energy gradient brings about an increase in the relative velocity within the moving-blade ducts formed between the moving blades. Where the blade forces are almost exclusively action forces in the constant-pressure turbine, in an excess-pressure turbine, a greater or lesser fraction resulting from the change in the velocity magnitude is added. The term "excess-pressure" turbine is derived from the pressure difference between the downstream and upstream side of the moving blade. In an excess-pressure turbine, therefore, a change in the velocity magnitude takes place when the pressure varies.
In a thermal turbo-machine, the percentage apportionment of the isentropic enthalpy gradient in the moving blades to the total isentropic enthalpy gradient by a stage having a guide-blade ring and moving-blade ring is designated as the isentropic reaction degree r. A stage in which the reaction degree r is equal to zero and the greatest enthalpy gradient occurs is designated as a pure constant-pressure stage. In the case of a classic excess-pressure stage, the reaction degree r is equal to 0.5, so that the enthalpy gradient in the guide blades is exactly the same as in the moving blades. For example, a reaction degree r equal to 0.75 is designated as a strong reaction. In steam-turbine construction practice, the classic excess-pressure stage and the constant-pressure stage are predominantly employed. However, as a rule, the latter has a reaction degree r that is somewhat different from zero.
Furthermore, the terms "chamber turbine" and "drum turbine" are used. Conventionally, a constant-pressure turbine employs a chamber configuration and an excess-pressure turbine employs a drum configuration. A chamber turbine has a casing that is divided into a plurality of chambers through intermediate floors disposed at an axial distance from one another. A disc-shaped rotor, on the outer periphery of which the moving blades are mounted, runs in each of these chambers, while the guide blades are inserted into the intermediate floors. One advantage of the chamber configuration is that the intermediate floors can be sealed off at their inner edge relative to the turbine shaft in a highly effectively manner through labyrinth gaskets. Because the gasket diameter is small, the gap cross-sections and, therefore, the gap leakage streams both become small. In known turbines, the configuration is used only in the case of low reaction degrees, that is to say a high stage gradient and, therefore, a small number of stages. The pressure difference on the two sides of a rotor disc is small in the case of a low reaction degree and, in the borderline case, is even zero. An axial thrust exerted on the rotor remains low and can be absorbed by an axial bearing.
In a drum turbine, the moving blades are disposed directly on the periphery of a drum-shaped turbine shaft. The guide blades are inserted either directly into the casing of the steam turbine or into a special guide-blade carrier. The moving blades and guide blades may also be provided with covering strips, to which labyrinth gaskets are attached, so that a sealing gap between the guide and moving blades and the turbine shaft and inner casing, respectively, is sealed off. Because these sealing gaps are located on large radii, at least in the case of the moving blades, the gap leakage streams are at all events considerably greater than in the case of chamber turbines. Due to the higher reaction degree, for example, r equal to 0.5, favorable flow paths in the blade ducts and, therefore, high efficiencies are achieved.
The axial overall length and the outlay for an individual stage are less than in a chamber turbine, but a larger number of stages is required because the reaction stages process a lower gradient. The axial thrust occurring in the blading is considerable. One possibility for counteracting the axial thrust is to provide a compensating piston, to the front side of which the pressure of the outlet port is applied through a connecting conduit.
A steam turbine of the drum configuration is described in German Published, Prosecuted Patent Application 20 54 465, corresponding to U.S. Pat. No. 3,754,833. A turbine shaft carrying the moving blades and an inner casing surrounding the turbine shaft are disposed in a pot-shaped outer casing. The inner casing carries the guide blades. The inner casing is connected to the outer casing through corresponding bearing and centering points for the absorption of an axial thrust.
A multi-stage steam turbine having high-pressure, medium-pressure and low-pressure turbine sections is described in U.S. Pat. No. 1,092,947 to Pape. The individual turbine sections are disposed in a single casing. The high-pressure section, which is a single stage, has a stationary guide blade that is disposed between two moving-blade rows disposed on a common wheel disc. The high-pressure section is, therefore, not a chamber configuration or a drum configuration. The medium-pressure section has a chamber configuration and the low-pressure section has a drum configuration. In a second embodiment, the low-pressure section is of the double-flow configuration.
A steam turbine having a high-pressure and a medium-pressure o turbine section is disclosed in U.S. Pat. No. 1,750,814 to Pape. The high-pressure turbine section has a drum configuration and the medium-pressure turbine section has a chamber configuration. The two turbine sections both may be disposed on a single shaft, or alternatively, on a separate shaft, and are each disposed in their own casing and are fluidically connected to one another. The high-pressure section has excess-pressure blading or constant-pressure blading.
A combined drum and disc-wheel turbine for steam, in which the last stage of the turbine is configured with disc wheels (chamber configuration), is specified in German Patent No. 448247. The entire steam turbine, including the section having a drum configuration and a section having a chamber configuration, is disposed in a single turbine casing.